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Pot experiments were conducted to study the variation and physiological regulation of transpiration efficiency (TE) of four winter wheat (Triticum aestivum L.) varieties that are widely grown in different ecological regions in North China. Plants were grown under two soil moisture regimes, normal and drought stress. The results showed that under drought stress condition, both TE at plant level and TE at leaf level (TEl) increased significantly. The transpi ration rate (Tr) was reduced more strongly than leaf net CO 2 assimilation rate (Pn). The decline of Tr was mainly affected by stomatal conductance and the decline of Pn was affected by non-stomatal factors, which was confirmed by the decline in net photosynthetic oxygen evolution rate. The leaf soluble sugar content and proline content were significantly increased under drought stress. The stomatal density was increased and the stomatal length was re duced. These results led us to make the following conclusions: (1) Under drought stress, the increase in TEl appears to be regulated in two ways: via the stomata by regulating Tr, and independent of the stomata through regulation of Pn; regulation via the stomata was more sensitive; (2) Osmotic adjustment was closely correlated to the non-stomatal regulation, and stomatal aperture was closely correlated to the stomatal way.

An open field experiment was conducted under furrow irrigation with 3 treatments: CK (control), PRD (partial root drying) and RDI (regulated deficit irrigation). The results showed that water potential, water content of the leaf and growth were decreased under PRD and RDI and the plants met stronger water stress under RDI than under PRD regime. The water use efficiency (WUE) based on fruit yield reached to 10.95 kg/m 3 and 15.33 kg/m 3 , i.e. 17.1% and 63.9% increase over CK under RDI and PRD, respectively. The transpiration efficiency in RDI was kept at the same level as CK, whereas it was promoted by 32.4% under PRD condition. CAT, SOD and POD activities were more active under RDI and especially under PRD than under CK. Therefore, following conclusions could be made: mode rate water stress induced osmotic regulation under PRD conditions, leading to normal water status, higher antioxidant enzymes activities, the same level of biomass and lower water use, thus providing some part of mechanism to higher WUE under PRD condition.